I. Bertóti

Characterization of nitride coatings by XPS

Nitride coatings have been used in numerous applications to increase the hardness and improve the wear and corrosion resistance of structural materials, as well as in various high-tech areas, where their functional rather than mechanical properties are of prime importance. Performance of these coatings is equally dependent on their chemical composition and long-range crystalline structure, as well as on the nature and amount of impurities and intergranular interactions. Significant improvement in the mechanical properties has recently been achieved with multi-component superlattice and nanocomposite nitride coatings. In the case of such multi-component systems, not only is close control of the elemental composition (stoichiometry) necessary to optimize the properties of the coatings, but the influence of chemical bond formation between the components is also of prime importance. Special care needs to be taken when non-equilibrium preparation conditions, activation of CVD and PVD by plasmas or energetic particle beams are applied, occasionally leading to unpredicted deviations, both in composition and structure. As is highlighted in this paper, nitride coatings or nitrided surfaces can be analyzed in detail by X-ray photoelectron spectroscopy (XPS) due to its excellent element selectivity, quantitative character and high surface sensitivity. More importantly, XPS reflects the atomic scale chemical interactions, i.e. the bonds between neighboring atoms, and thus it also provides reliable structural characteristics for amorphous or nano-crystalline coatings of complex composition, for which application of diffraction techniques is not straightforward. A number of examples of the application of XPS are given for various types of nitride coatings, including interstitial compounds, such as TiN, CrNx, etc., as well as compounds with predominantly covalent bonding, such as AlN, GaN, Si3N4 and CNx. Special emphasis is placed on ion beam-induced compositional and structural changes and to the formation of ‘superstoichiometric’ TiN1+x, ZrN1+x compounds.

Surface characterisation of plasma-nitrided titanium: an XPS study

DC plasma nitriding was applied to titanium metal sheets in a commercial cell using Ar + NH3 as the admixture and the nitrided surface investigated by means of XPS. As a comparison, in situ nitriding of a chemically pure Ti surface at room temperature was performed by N2+ ion bombardment (1–5 keV) in the electron spectrometer. Synthesis of the complex Ti2p envelope was accomplished using two sets of loss peaks, separated by 1.6 and 3.0 eV from the major TiN-type Ti2p32 and Ti2p12 components. A doublet at 458.8 and 464.5 eV was also included to account for a TiO2-type oxide. The sum of the main Ti TiN peak and the two loss peaks were taken to be representative of nearly stoichiometric TiNx with x ranging from 0.85 to 1.15. Further components derived from the peak synthesis were assigned to TiNxOy and Ti2O3. The stoichiometric nitride is represented by a Ti2p doublet at binding energies of 454.7 and 460.6 eV and a single sharp N 1s peak at 396.7 ± 0.1 eV. On the superstoichiometric samples, especially after N2+ bombardment, a second peak appears at about 395.8 eV with a positive correlation between this peak concentration and the relative amounts of species (TiNxOy, Ti2O3, TiO2) derived from Ti2p components and the surface O and N content. Consecutive Ar+, N2+ and (N2+ + O2+) bombardment leads to significant changes in composition together with rearrangement of short-range chemical structure which is reflected in peak-shape changes of the Ti2p and N 1s lines.

Surface modification and characterization of particulate mineral fillers

Abstract The efficacy of the surface treatment of particulate fillers depends on the chemical character of the components, on the method and conditions of the treatment, and on the amount of treating agent. To achieve maximum efficacy the coupling agent must be chosen specifically for each polymer/filler pair. Because of its ionic character CaCO3 can be effectively treated with stearic acid, which forms a basic salt on the filler surface. One stearic acid molecule is attached to each Ca2+ ion and in the complete monolayer the molecules are vertically oriented to the surface. Maximum efficacy is expected at the monomolecular coverage of the surface, but the methods utilized to determine this concentration yielded different results. Stearic acid surface treatment of CaCO3 decreases its surface tension and the effect is more pronounced on the polar than on the dispersion component. The change in the surface characteristics of the filler influences also the properties of the composites in which they are used.

Effect of ion and neutral sputtering on single crystal TiO2

Abstract Alteration of the structure and composition of solid surfaces under energetic ion or neutral particle impact is a major concern in surface analysis, mainly in connection with depth profiling. It has been established that preferential sputtering occurs in many compounds and systems and that this leads to chemical transformations. Titanium compounds, especially the oxides, possess various stoichiometric and non-stoichiometric states and thus offer a suitable vehicle for investigating these effects. This paper reports studies on the reduction of single crystal TiO 2 by He, Ar and N 2 ion and fast atom bombardment. Attempts have been made in this study to comprehensively characterize the chemical and electronic state of the ion or neutral bombarded single crystalline TiO 2 surfaces by X-ray Photoelectron Spectroscopy (XPS). Differences observed in the extent of bombardment induced reduction due to various incident species (He, He + , Ar, Ar + , N 2 , N 2 + have been discussed and explanations of these differences in terms of a compensating electron leakage current have been proposed.

Formation of boron nitride thin films on β-Si3N4 whiskers and α-SiC platelets by dip-coating

Abstract Thin films of boron nitride have been deposited on the surface of β-Si 3 N 4 whiskers and α-SiC platelets by dip-coating in aqueous and methanolic boric acid solutions, respectively followed by drying at room temperature, heating in nitrogen flow, and finally nitriding in an ammonia flow at 1000 °C. X-ray photoelectron spectroscopy (XPS), infrared (IR) spectroscopy and scanning electron microscopy (SEM) studies proved the formation of boron nitride surface coating. The XPS studies indicated that the methanolic boric acid solution was a more effective precursor as compared to the aqueous solution. Transformation of H 3 BO 3 to BN was much more efficient on the surface of SiC platelets than on that of Si 3 N 4 whiskers.

Thermal plasma synthesis of zinc ferrite nanopowders

Ž. Nanosized zinc ferrite spinel powders of various compositions were produced in a radiofrequency RF thermal plasma from oxide mixtures and from co-precipitated hydroxides. Bulk and surface chemical compositions of the products were measured by ICP-AES and XPS. Phase conditions were determined by XRD. Morphology was investigated by SEM and TEM. In spite of the short residence time of reagents in the plasma reactor, a high degree of spinel formation was achieved. The Zn enrichment on the surface determined by quantitative XPS is attributed to a thin ZnO layer on the surface of the zinc ferrite particles. In zinc ferrous ferrites, an inhomogeneous distribution of Zn inside the grains may also exist. q 2001 Elsevier Science B.V. All rights reserved.

Response of oxides to ion bombardment: the difference between inert and reactive ions

Abstract The loss of O from oxides bombarded by chemically neutral ions, i.e. inert gas ions, is widely documented. It is, however, still not possible to relate with certainty such loss to the basic properties of the oxides, namely to such factors as mass, surface binding energy, bulk binding energy, volatility, mass transport, Gibbsian segregation, or electric fields. Even the most basic response of all, namely chemically random behavior [Vertoti et al., Surf. Interf. Anal. 19 (1992) 291), has an unestablished importance. The present work further develops our approach to the problem. The effects of 1–5 keV bombardment with Ar + , O 2 + , N 2 + , and N 2 O + are compared using X-ray photoelectron spectroscopy for a number of oxide pairs with similar stoichiometries and electronic structures: B 2 O 3 and Al 2 O 3 , SiO 2 and GeO 2 , ZrO 2 and TiO 2 , V 2 O 5 and Nb 2 O 5 . Minor O loss occurred with the first five and major loss with the last three. What is important, however, is that the loss was consistently higher for N 2 + than Ar + impact, yet the overall anion content, expressed as the sum O + N, remained largely unchanged. We take this result as demonstrating a prominent role for chemically random behavior, i.e. the relative strengths of the MO and MN bonds did not enter.

Surface treatment of polyethylene by fast atom beams

Abstract Ultra-high molecular-weight polyethylene (UHMWPE) was treated by fast atom beams (FAB) obtained from He, Ar, H 2 and N 2 with about 1 kV accelerating voltage and estimated fluence of 10 17 particles cm −2 . The modified surface layers were characterised by X-ray photoelectron spectroscopy (XPS or ESCA) and dynamic microhardness measurements. Each applied FAB treatment results in the increase of the bulk plasmon loss energy ( E L ) of the C 1s peak. This implies the formation of graphitic-type material and/or hydrogenated amorphous carbon (or carbon nitride in case of treatment by N atoms) in the modified surface layer. FAB treatment by N atoms leads to the incorporation of ≈35 at.% of N, which is about three times higher than the value obtained previously after low-kilo-electron-volt N 2 + ion-beam treatment. The related C 1s peak shows that the overwhelming portion of C is bound to N, while the N 1s peak reflects that N is present in at least three chemical bonding states. Angle-dependent XPS studies of the nitrogen-FAB-treated UHMWPE reveal the presence of a N-rich subsurface layer with a topmost layer containing less N. This is in agreement with the calculated depth distribution of N atoms. Each applied FAB treatment leads to significant increase in the surface microhardness.

Surface characterization of ultrahigh molecular weight polyethylene after nitrogen ion implantation

Ultrahigh molecular weight polyethylene (UHMWPE) samples were implanted with 46 and 80 keV nitrogen ions up to a fluence of 10 17 ions cm -2 . The modified surface layers were studied by XPS, Fourier transform infrared spectroscopy (FTIR), Rutherford backscattering spectrometry, elastic recoil detection analysis and dynamic ultra-microhardness measurements. By XPS at relatively low fluences (∼10 15 ions cm -2 ) the dominant N 1s peak component was detected at ∼400 eV, which was attributed to C=N-type bonds. With increasing fluence the share of component at ∼398.5 eV, assigned to C-N-type bonds, increased significantly. Changes in the FTIR spectra reflected dehydrogenation (creation of trans-vinylene groups) and oxidation (creation of carbonyl groups). Rutherford backscattering spectrometry allowed the depth profiles of the elements to be obtained. In each case, the thickness of the oxygen-containing layer proved to be greater than that of the nitrogen-containing layer. Elastic recoil detection analysis revealed the formation of a layer with a characteristically graded hydrogen depletion. The thickness of this layer was greater than the projected range of the nitrogen ions. Significant improvement in the surface hardness was observed for the ion-implanted UHMWPE in the whole range of indentation depth studied.

Kinetics of γ-alumina chlorination by carbon tetrachloride

Abstract The kinetics of the reaction between γ-Al2O3 and gaseous CCl4 has been studied by isothermal TG measurements in the temperature range 700—1123 K. The reaction starts with a weight gain which can be attributed to the chemisorption of the reactive gas. The weight loss vs. time curves at relatively high temperatures can be described by the contracting cylinder equation and at relatively low temperatures by first-order kinetics. The dependence of the initial reaction rate on the CCl4 partial pressure follows the Langmuir—Hinshelwood rate expression. At 700—723 K, chemical control is thought to be predominant and an apparent activation energy of 212 kJ mole−1 is found for the chlorination process.